Low-pressure mercury gas discharge lamps are widely used in the field of lighting, but also in the field of disinfection because of their pre-dominant output of ultraviolet radiation, which has a germicidal effect. In disinfection applications, the terms “UV lamp” and “UV radiator” are used as equivalents for high-output low-pressure mercury gas discharge lamps. These terms will be used in the present specification as well.
While in lighting applications, the main requirements are a balanced spectrum in the visible wavelength range, a high efficiency of the light output in relation to the electric power consumption and a long service life, the requirements of ultraviolet (UV) radiators are different. The ultraviolet output has to be very high because the intensity of the ultraviolet radiation is directly correlated with the disinfection efficiency, which means that fewer UV radiators with a higher UV output can be used for disinfecting water, and that directly reduces the investment necessary for drinking water or wastewater disinfection installations. Another important requirement of UV radiators for disinfection purposes in larger installations is the total power consumption. In drinking water and wastewater applications, the volume of water per time unit, (i.e. cubic meters per second) can vary significantly. In order not to waste unnecessary amounts of ultraviolet radiation and the electric energy associated with its production, several techniques have been developed to adapt the output of UV plants to the water flow. There are solutions in which the water is treated in several parallel channels, each being equipped with a number of UV radiators, and in which individual channels can be closed when the water flow is low. Other applications provide for the possibility to reduce the electric power input of the lamps and consequently dim the lamps to a lower UV output when the water flow is low. Dimming UV lamps of the low pressure mercury type is limited to about 30% of the nominal power output because the filaments at the ends of the lamp are heated by electric discharge in the lamp, and if the electric power supplied to the discharge is reduced, the temperatures of the filaments are also reduced. At a certain point, the filaments get too cold to provide the necessary electron emission. There is the risk that the lamp ceases to function, but also the risk that the filament is damaged when it is operated at too low temperatures. Therefore, there is a lower limit for the electric input of UV lamps.
For lighting purposes, there are several prior art documents known which use more than one filament at each end of the lamp. Such prior art documents are Chinese patent application CN 1812677 A and Chinese patent CN 101644389 B, and U.S. Pat. No. 6,756,745 B1. These lamps are used for lighting purposes and suggest multiple filaments, which are redundant in the sense that, if one filament is defective, the other filament can be switched on. There is no disclosure that the filaments can be of different size and mass. As discussed above, the technical challenges in lighting applications are different from those in ultraviolet disinfection applications.